Conventional oil field casing may be used as the drillstring for drilling oil and gas wells to simultaneously drill and case the wellbore. Once the wellbore is drilled to the desired depth, the casing is cemented into the earth without withdrawing it from the wellbore. A retrievable drilling assembly, including a bit and borehole enlarging tool, may be attached to the bottom end of the casing for drilling. This drilling assembly often includes other components such as mud motors, MWD collars, LWD collars, non-magnetic drill collars, steel drill collars, and stabilizers.
Once the casing is drilled to the desired casing setting depth, the drilling BHA is retrieved from the casing with a wireline before the casing is cemented in place. In some cases the BHA must be retrieved and replaced before the casing is drilled to its terminal depth, for example to replace a worn drill bit or to replace some other failed component in the BHA. A provision must be made for the wireline to be run through the casing to retrieve the BHA. It is often advantageous to circulate drilling fluid down the ID of the casing while the wireline is being run and the BHA recovered to assure that any influx of formation fluids is circulated out of the well in a controlled manner. It is also advantageous to reciprocate the casing while the BHA is being recovered so that the casing does not become stuck in the borehole. A top drive and a casing drive system may be used to support the casing to permit circulation and reciprocation of the casing. A top drive is often used to rotate the casing for drilling. The casing may be attached to the top drive with a casing drive system that grips the top of the casing without screwing into its upper threaded connection. The casing drive system also provides seals so that drilling fluid can be circulated down the inside diameter of the casing to flush cuttings away from the drill bit and up the annulus between the casing and the borehole wall.
The drilling rig used to drill with casing may be a specially designed rig that facilitates the efficient operation of the wireline for running and retrieving the drilling BHA. The rig also must be equipped with a wireline unit that is capable of handling the drilling BHAs. For rigs designed for casing drilling, this wireline unit may be provided as an integral part of the rig.
Access for the wireline is provided through the top of the swivel, which may be incorporated as an integral part of the top drive. The wireline access through the top of the swivel may be facilitated by utilizing a split crown block and split traveling block. Split blocks are ones where the sheaves used for carrying the drilling line are divided into two groups spaced laterally apart. The split crown arrangement allows a wireline sheave to be hung at the crown of the rig so the wireline can be aligned with the central axis of the drillstring. The split traveling block provides room for a wireline stripper assembly and wireline BOP to be attached to the top of the swivel to prevent the pressurized drilling fluid from escaping around the wireline as it is being run into and pulled from the casing. In some situations, it may be sufficient to provide only a split traveling block as the fleet angle from having the crown sheave offset slightly from the central axis of the drillstring.
The drilling BHA may be quite heavy and weigh as much as 30,000 pounds. A large braided cable, for example ¾″ in diameter, may be required to support this much weight and the sheaves used with such a cable are relatively large in diameter, for example 30″ in diameter. It is important that the sheaves and wireline pressure control equipment be positioned so that the wireline can enter the casing along its central axis. Otherwise, the cable will exert lateral forces on the casing or other equipment and will quickly cut into the equipment as it is run into and out of the well. In some situations, it may be advantageous to use a drilling rig that is designed specifically for drilling with casing when one is available. Often such a rig may not be available or only a portion of the well may be drilled with casing so that it may be more convenient to use a conventional rig.
There are only a few drilling rigs in the current fleet of rigs available for use in drilling oil and gas wells that are equipped as described above for using casing as the drillstring. It is possible to modify any drilling rig to include the facilities needed to easily handle the wireline when drilling with casing. Most conventional drilling rigs do not include split crown blocks and split traveling blocks to facilitate wireline access along the central axis of the drillstring. The time required to modify the rig to accommodate these parts and the capital cost of the modifications may not be justified when the rig is used to drill only a portion of a well with casing. Furthermore, the owner of the rig may not allow structural changes to be made to the rig. This is particularly true for expensive offshore rigs.
There are devices described in the prior art for providing wireline access to the ID of a drillstring. For example, U.S. Pat. No. 6,202,764 describes a wireline entry sub that can facilitate wireline use on a rig. Although such a wireline entry sub has been described, it is desirable that an improved sub be provided.
For example, in some cases, particularly for offshore rigs, the space for placing a wireline reel and tensioning unit near the drilling floor may be limited. Therefore, the wireline unit may be placed away from the rig floor. However, the wireline, which may be operating under a high-tension load, may be a hazard to rig personnel moving about the area.